Looks like MR15 at the Swaziland / South Africa border has been closed since the late 1980s due to floor damage to the road.
[Here is a recent trip log][1] by the 4x4 enthusiast group "Jeep Freaks SA" who visited travelled the South African side upto up to the Swaziland border. The road has been made impassable by for motor-vehicles at the border, but looks like it is in use by people on foot.
"The area bordering Swaziland and Mozambique between Ingwavuma and the Ndumo Game
Reserve was also identified as a smuggling route for firearms in the past. It is still in use by
gunrunners. The area is very difficult to patrol due to the mountainous terrain and lack of
suitable roads. The local population make no secret that they move freely through the Cecil
Mack’s pass area and that criminals also use these routes" - [Monograph 57: The Challenge to Control South Africa`s Borders and Borderline, Ettienne Hennop, Clare Jefferson and Andrew Mclean][2]
[1]: https://www.facebook.com/permalink.php?story_fbid=426040440924504&id=321438548051361
[2]: https://issafrica.org/research/monographs/monograph-57-the-challenge-to-control-south-africas-borders-and-borderline-ettienne-hennop-clare-jefferson-and-andrew-mclean
